This invention relates generally to a cylinder head of aluminum alloy for an internal combustion engine, including a head member having a bottom wall forming a combustion chamber wall and having a pair of spaced ports for the intake and exhaust valves and an opening for an injection nozzle and/or an ignition aid, the bottom wall having a sealing surface which delimits the combustion chamber wall, and the head member including a wall portion defining a bridge between the spaced ports.
The stress of the cylinder head in the area of the combustion chamber wall and in the sealing surface region between the cylinder head and the cylinder barrel, makes two contradictory demands on the cylinder head material. The bridge between the gas exchange valves must have an increased resistance to thermal shock and deformation, whereas the sealing surface must retain a high residual hardness (compressive creep strength) at elevated temperatures.
In today's high-performance engines, temperatures from 280.degree. to 300.degree. C. are present at the wall portion of the cylinder head which defines a bridge between the intake and exhaust ports. When all constructional measures to optimize the cooling have been exhausted, deliberate upratings of internal combustion engines produce a higher thermal load on the cylinder heads, particularly in the area of the bridge between the intake and exhaust ports. Heat resistance alloys of cast aluminum which have been used can no longer withstand these temperatures for long periods of time especially when the thermal stress of the combustion chamber exceeds 300.degree. C. As shown by German published application No. 142 6 122, it is known to introduce inserts of a material having a high heat resistance, such as steel or a cast high-alloy graphite, into the bottom wall or lid portion of the cylinder head forming the combustion chamber wall so as to increase the thermal loading capability thereof. However, due to different thermal expansions of the inserts relative to that of the cylinder head, the bond therebetween deteriorates after a period of time and results in serious damage of the engine. Moreover, the weight of the aluminum cylinder head must be appreciably increased in order to render it compatible with such inserts.
And, in the regions of the sealing surface, particularly at the exhaust port area, high temperatures effect a considerable reduction in the hardness which had been achieved through precipitation hardness aging, even for those alloys of cast aluminum known for their high-temperature strength. As a result, the compressive creep resistance of the cylinder head in the region of the sealing surface lessens and results in leakage between the cylinder head and the cylinder barrel. Inserts of other materials cast into the region of the sealing surface or mechanically fastened therein, as set forth in German published applications No. 205 9 219 and 283 8 797, possess the same drawbacks as discussed with reference to the aforementioned German published application.